It seems safe to say that tablet sales have peaked.
According to the latest preliminary figures from $4, the global tablet market shipped 36.2 million units in the first quarter of 2017, a decline of 8.5% year-over-year. As this chart from $4 shows, that is the lowest total since the third quarter of 2012, and the tenth straight quarter of declining growth.
As IDC notes, the drop here is coming from traditional "slate" tablets, like the standard iPad. Those appear to have dipped for a number of reasons - the rise of big-screen smartphones, the rise of "convertible" touchscreen laptops, samey designs, certain tablets being good and/or not-used-enough to require regular upgrading, and so on.
On the other side are "detachable" tablets, like the iPad Pro or Microsoft's Surface Pro, which come with a keyboard. Those are growing with each passing quarter, but they tend to be pricey. We'll soon see if the launch of $4 affects things, but for now, the tablet seems to have settled into a spot of influencing the PC more than supplanting it entirely.
Mike Nudelman/Business Insider/Statista